Backside damaging of silicon semiconductor wafers to provide sources of extrinsic gettering of metallic impurities has been accomplished by several methods including liquid honing, striated extrinsic gettering, dry blast abrasive powder, phonographic techniques, spiral particle paper abrasive, laser technology, and poly silicon growth.
Liquid honing and dry blast abrasive powder systems are usually a problem from a controllability standpoint. This is due to the extremely small abrasive particle size. Smaller abrasive particles tend to agglomerate or "clump" causing clogging of abrasive lines which makes this type of backside damage inherently unstable. Lateral cracks and pits produced by this type of backside damage makes the wafer difficult to clean particles from the damaged side. The particles cause particulate wafer contamination after the polishing step which can be detrimental to wafer yields. The silica abrasive used with the dry blast system has also been known to cause silicosis under prolonged exposures.
Striated extrinsic gettering, phonographic techniques and spiral particle paper method all produce damage to silicon wafers by producing deep scratches of the wafer surface. Scratching the surface of silicon causes pits and lateral cracks in the wafer surface. A pit is formed in silicon wafers by "knocking out" pieces of silicon on the surface level. This dislodged silicon then plagues the wafer process later in the form of particulate contamination. The phonographic technique is almost unusable due to low product throughput. This technique uses a diamond tipped stylus to damage a spinning wafer by moving the stylus slowly from the center of the wafer outward producing a spiral pattern of damage.
The spiral particle paper method to backside damage wafers is not widely used due to low production throughput. This method uses a sand paper like strip to abrade the wafer by contacting the surface of the wafer with the abrasive strip and spinning the wafer in a circular path. This form of abrading leaves a mound at the center of the wafer. Polishing the wafer later produces a dimple at the center of the spiral pattern on the front surface of the wafer due to the mound on the backside of the wafer.
Laser backside damage has been pursued as an alternative to abrasive brushes, powders, and papers. Silicon is melted at the surface of the wafer which causes a poly silicon layer. The main problem with laser backside damage is during the recrystallization of the melted silicon. Melted silicon recrystallizes forming a crystal structure which resembles the original material. This is undesirable since this "erases" most of the poly silicon nature developed immediately after melting.
Poly backside damages is about the best type of backside damage known. Unfortunately it is also the most expensive and time consuming. Poly silicon is grown on the backside of wafers. This method is excellent from a particle standpoint since the backside is never subjected to abrading. Price is the major drawback to the method of backside processing.